三江地区义敦岛弧安山岩成因

Origin of the Andesite in Yidun Island Arc,Sanjiang Region

义敦岛弧是一个发育于张性薄陆壳基底上的三叠纪岛弧,以发育岛弧裂谷和成对的火山弧为特征。弧火山岩主要为钙碱性火山岩系,以安山岩为主,其岩石组合为玄武岩—安山岩—英安岩。岩相学、地球化学、相平衡及定量模型计算均证实,分离结晶作用是产生钙碱性火山岩系的主导作用,安山岩是钙碱性玄武岩浆在不同压力条件下发生多阶段分离结晶的产物。岩浆演化具有相对封闭系统特征。玄武岩浆在较大压力条件下发生Ol+Cpx+Pl±Mt±Ap结晶(F=0.46)后,派生岩浆发生Hb+Cpx+Pl+Mt结晶分离(F=0.25),产生安山岩浆。该岩浆可能在较低压力下再次发生结晶分异,从而派生出英安岩。角闪石在相对较高的压力条件下的大量结晶是安山岩形成的主要原因。

The Yidun Island Arc is a Triassic tensional island arc developed at thebase of a thin continental crust, which was tensional in the Permian andthe earlier Triassic. It is characterized by development of intra-arc riftingzone and inner-and outer-arcs. Aic volcanic rocks are mainly calc-alkalinevolcanic rocks, consisting of basalt, basaltic andesite, andesite, dacite andpyioclastic rocks. Studies on their petrography, major and trace elementgeochemistry and phase equilibrium have shown that the calc-alkalinevolcanic rocks Were fomed by fractional Crystallization of a calc-alkalinebasaltic magma and that the andesite was formed by multi-stage crystalli- ??zation of calc-alkaline basaltic magma under different pressures. Modelcalculations show that after Ol+Cpx+Plag±Mt±Ap crystallization fromcalcalkaline basaltic magma under higher pressure, fractional crystallizationof Hb+Cpx+Plag+Mt took place in derived magma, thus producing andesitemagma. So, separation of a large quantity of Hb mineral is an importantfactor controlling the formation of andesite.